Final answer:
Proposition 21 was a controversial ballot initiative that aimed to make it illegal for undocumented aliens to receive public program benefits. However, a federal court ruled the measure unconstitutional.
Step-by-step explanation:
Proposition 21 refers to a controversial ballot initiative that was approved by California voters. It aimed to make it illegal for any undocumented alien to receive the benefit of public programs, such as schools and health clinics.
However, a federal court determined that the measure was preempted by federal laws regarding the creation and enforcement of immigration law, leading to the proposition being deemed unconstitutional.